1. Field of the Invention
The invention relates to an x-ray examination apparatus for forming an xray image by irradiating an object, said apparatus including an x-ray image intensifier for converting said x-ray image into a visible image and an image detection system which incorporates a light-sensitive device to convert said visible image into an electronic image, a read-out circuit for reading out said electronic image by transforming said electronic image into an electronic video signal, and a control-circuit for the read-out-circuit. The invention also relates to such an image detection system.
2. Description of the Related Art
An x-ray examination apparatus of said kind has been described in the U.S. Pat. No. 4,881,124.
There are several operating modes for performing x-ray radiology. In particular, a patient to be examined is exposed to a low dose of x-rays continuously or in a series of pulses during fluoroscopy, whereas pulses of a higher dose of x-rays are applied during exposure. An x-ray image intensifier is provided to convert an x-ray image into a visible image which is subsequently detected by a light-sensitive device incorporating a light-sensitive element. Such a light-sensitive element is substantially sensitive to light having a wavelength in a range covering the wavelengths between ultra-violet radiation and infrared radiation. In the x-ray examination apparatus of the cited reference, the light-sensitive element is a photoconductor-type image pick-up tube. When a sequence of x-ray images is made, time-resolution of such a sequence is determined by the image frequency, which itself is determined by the read-out rate of the light-sensitive device. The control circuit is arranged to adjust vertical and horizontal deflection frequencies of the scanning electron-beam of the pick-up tube. The control circuit is further arranged to adjust the scanning electron-beam current. The xray examination apparatus of the cited reference comprises an operating mode selection means. The control circuit adjusts the image pick-up tube in accordance with the selected operating mode, so as to provide proper image frequency and image quality. The control circuit of the known x-ray examination apparatus is not suitable for use together with a solid-state image sensor, notably a charge-coupled device, because such solid-state image sensors do not employ a scanning electron-beam to read out an image. Moreover, said adjustments relate to the entire area of the light-sensitive face of the photoconductive element of the image pick-up tube. Consequently, a control-circuit of the known apparatus is not capable of making adjustments which relate to specially selected parts of images.